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Gilead's Sciences' idelalisib will be used in combination with Roche and Biogen Idec's Rituxan. An FDA approval is key, with Infinity Pharmaceuticals hot on its heels.

Gilead Sciences' (NASDAQ:GILD) blood cancer drug idelalisib is up for FDA approval for two different indications. The biotech giant should hear about the application it submitted second on or before Aug. 6. It's not scheduled to hear about the first application until September 11.

Confused? Welcome to the world of priority reviews.

The first application, submitted back in September was for patients with indolent non-Hodgkin's lymphoma, or iNHD, who didn't respond to previous treatment. That indication was given a standard 12-month review.

Then in December, Gilead Sciences submitted an application to use idelalisib in previously treated chronic lymphocytic leukemia, or CLL. That application was given an eight-month priority review, thus jumping ahead of its sister application.

While it isn't necessarily a judgment on the approvability of idelalisib in one indication or the other, by giving the drug a priority review for CLL, the FDA is essentially saying idelalisib can have more of an impact on CLL patients than it can on iNHL patients.

And for good reasonA phase 3 trial in previously treated CLL patients that weren't fit for chemotherapy -- CLL primarily affects the elderly -- was stopped early because it was clear to the data monitoring committee that idelalisib plus Roche and Biogen Idec's (NASDAQ:BIIB) Rituxan was better than Rituxan alone. The combination increased progression-free survival, overall response, lymph node response, and, most important of all, overall survival. In fact, there was a 72% reduction in the risk of death when idelalisib was added to Rituxan compared with Rituxan alone.

Gilead seems to have thrown Roche and Biogen Idec a bone here because idelalisib will be used in combination with Rituxan rather than instead of it. In these highly treated patients, it's not clear that Rituxan is really doing anything.

It appears that Gilead is thinking ahead, though. While the combination might not be all that useful for late-stage patients, Gilead eventually wants to treat patients earlier in their disease progression -- a phase 2 trial in previously untreated patients in under way. It's more likely the combination in those patients would do something, especially since the two drugs work through different mechanisms.

Approval likely?You can never say with certainty that a drug will be approved, but there are plenty of clues that the FDA will green-light idelalisib in late-stage CLL patients.

Getting a priority review isn't necessarily based on the data, but it does signal that the FDA thinks there's an unmet need, which can make it more likely the FDA will overlook issues like side effects. Idelalisib also has Breakthrough Therapy designation for CLL, which means the FDA likes what it saw during the superficial data review for the designation.

An approval to treat iNHL in September also looks fairly likely. The approval is based on a phase 2 open-label, single arm trial, which is always dangerous interpreting since there isn't a control group. Still 57% of patients responded to the drug with more than half of the responses lasting longer than a year. That doesn't just happen spontaneously.

It's important for Gilead Sciences to get an FDA approval on its first attempt because Infinity Pharmaceuticals(NASDAQ:INFI) is hot on its heels with IPI-145, a drug in the same class as idelalisib. Infinity Pharmaceuticals could have data from its phase 2 registration trial in iNHL and a phase 3 trial in CLL next year. Getting into doctors' medicine bags now will help Gilead fend off Infinity Pharmaceuticals, especially if it isn't clear if IPI-145 is better than idelalisib, which unfortunately is often the case when comparing data from different trials.